1. Field of the Invention
The invention pertains to chucks used with machine tools, commonly known as collets, to hold a rotating workpiece between resilient fingers frictionally forced into a gripping relationship with the workpiece.
2. Description of the Related Art
Collets are commonly used with machine tools wherein the collet is mounted within a rotating spindle and the resilient collet fingers are radially translated between workpiece gripping and workpiece releasing positions by the axial displacement of the collet or the collet operator. The ability of the collet to quickly grasp and release a workpiece, and accurately hold the workpiece during machining makes this type of chuck highly desirable in many machine tool applications.
The collet fingers are radially forced inwardly by a camming action resulting from the engagement of a conical surface defined on the exterior of the collet fingers with a conical bore defined in the rotating machine tool spindle. Axial displacement of the collet relative to the conical spindle surface opens and closes the fingers to provide a frictional grasping of the workpiece. The gripping surface of the collet fingers is shaped complementary to the exterior surface of the workpiece, and collet gripping bores may be cylindrical, triangular, rectangular or polygonal, as necessary to accommodate the particular configuration of the workpiece as shown in U.S. Pat. Nos. 2,521,099 and 3,136,561.
It is known to improve the gripping surface of the bore defined by the collet workpiece gripping surfaces by serrating, etching, or otherwise configuring the workpiece engaging surface as shown in U.S. Pat. Nos. 1,955,642; 2,214,241; 2,228,337 and 2,595,828.
When performing secondary operations on previously machined or formed workpieces, it is often desirable that the workpiece be chucked within a rotating spindle of a machine tool by a collet. In such secondary operations, it is necessary to place the workpiece between the collet fingers from the front of the collet, which is usually the reverse direction of movement with respect to the workpiece and collet during primary machining operations wherein the workpiece stock is usually fed through the rear of the collet for access by the tools.
When placing a workpiece within a collet from the front, as preliminary to performing a secondary operation, relative axial movement between the workpiece and collet is necessary to insert the workpiece between the collet fingers. If the workpiece has a non-circular periphery, such as of a square or hexagonal configuration, which is common, it is necessary that the exterior configuration of the workpiece be properly aligned with the complementary collet bore in order to permit reception of the workpiece between the collet fingers. Accordingly, in past practice, it has been necessary to terminate the machine tool spindle and collet rotation prior to front loading the collet with such a workpiece. After the stationary collet and workpiece are properly related and the workpiece is located between the collet fingers, the collet fingers are closed and the spindle and collet rotation initiated in order to perform the secondary machining operation. Such continual stopping and starting of the machine tool spindle and collet is time consuming, but, previously, no collet configuration has been available which would permit the front loading of non-circular workpieces during collet rotation in order to reduce machining cycle time and machine tool wear.